1. Field of the Invention
The present invention relates to the preparation of aromatic carbamates by reacting at least one nitroaromatic compound, carbon monoxide and at least one organic compound bearing at least one hydroxyl group in the presence of a palladium-based catalyst.
2. Description of the Prior Art
The carbamates under consideration (or urethanes) are precursors of isocyanates, and the isocyanates are currently widely used, for example in the production of polyurethanes. Among the most important isocyanates, very particular mention will be made of diisocyanatotoluene (TDI) and methylenediphenyl diisocyanate (MDI), which hitherto have been produced industrially by phosgenation of the corresponding diamines. They frequently are in the form of mixtures of isomers.
Various processes have already been proposed to this art for reducing nitroaromatic compounds using carbon monoxide and a lower alkanol in the presence of various catalyst systems; all of which exhibit at least one of the following major disadvantages:
(1) presence of undesirable metal impurities in the reaction product; PA1 (2) high risk of corrosion of the apparatus due to the chloride ions originating, in most cases, from the ferrous or ferric chlorides employed as cocatalysts in the reaction. PA1 (i) either the molar ratio (H.sup.+ /Pd) is higher than 150, and/or PA1 (ii) the molar ratio (coordinate/Pd) is higher than 20.
More recently, a process has been proposed to the art for preparing aromatic carbamates by reacting (i) at least one nitroaromatic compound, (ii) carbon monoxide and (iii) at least one organic compound bearing at least one hydroxyl group in the presence of a palladium-based catalyst. In fact, such a process is described in European Patent Application EP-A-0,086,281, the distinguishing characteristic of which process comprising using particular classes of coordinates, typical representatives of which being tetraphenyldiphosphinoethane, 2,2'-dipyridyl and 1,10-phenanthroline, and which are used in practice in a proportion of 0.02 to 20 moles per gram-atom of palladium present in the reaction medium. In this same application, it is also proposed to carry out the reaction in question in the presence of an acid in order to increase the reaction rate and, if appropriate, the selectivity for the required carbamate. These acids are actually employed in a proportion of 0.01 to 150 equivalents per gram-atom of palladium present in the reaction medium.
However, while the principal advantage of a process of this type is not in dispute, its development on an industrial scale is inhibited by the relatively large amounts of palladium or of palladium compounds which have to be used in the reaction, in order to obtain an appropriate activity of the catalyst system and a acceptable product selectivity.
Notwithstanding the cost of the metallic catalyst, which necessitates the recycling thereof, and the versatile nature of said metal or its compounds, having to use large amounts of a metal of this type, or compounds thereof, makes it difficult to recycle it or them and increase the risk that the metal will be deposited, particularly on the walls of the equipment. Serious need therefor exists to carry out the reaction in question using restricted amounts of palladium or of palladium compounds. However, when such amounts are limited in order to attain at most a concentration on the order of 0.01 mole per liter, expressed in moles of palladium per liter of reaction medium, both a prohibitive decrease in the selectivity for the desired reaction product and a marked decrease in activity are realized.